This invention relates to insulated electrical conductors, and, in particular, to a method for forming insulated magnet wire and to a coil constructed from such insulated magnet wire.
Insulated magnet wire is typically used to form coils that create magnetic fields within electrical devices such as motors and transformers. In order to provide adequately strong magnetic fields, the coils are frequently manufactured with a multitude of overlaying layers of spirally wound wire. To ensure optimum operation of the coil, it is highly desireable that the electric current within the wire be prevented from jumping between successive windings (i.e. turn-to-turn jumping) in a coil layer, as well as prevented from jumping between successive layers (i.e. layer-to-layer) of the coil windings.
Overall, prior attempts to overcome layer-to-layer electrical current jumping have provided a workable degree of success. Such attempts frequently involved wrapping a sheet of electrically insulative material between the successive winding layers during coil manufacture. However, an assortment of existing techniques for preventing turn-to-turn electrical current jumping, while to a certain extent offering functional results, suffer from a variety of shortcomings which detract from their desirability.
For example, one technique previously employed to insulate a magnet wire is to coat the entire wire with an insulating resin or enamel. However, resin wire coatings alone are frequently susceptible to nicks or scratches during coil formation and/or use which compromises the insulating function of the coating.
An assortment of other techniques completely or partially wrap the wire with tapes or webs made of paper which absorbs oil that acts as an electrical insulator when used in an oil-filled transformer, or with tapes or webs made of insulating material, such as woven glass or NOMEX.RTM., which are less susceptible to nicks. However, due to the relatively high cost of the tapes such as made from NOMEX.RTM., these techniques, especially if excessive amounts of tape are utilized, make the wire product expensive. For example, one prior art technique spirally wraps the wire in a first direction with a first tape and then helically or spirally wraps the wire in a second direction with a second tape. While the second wrap may serve to stagger the tape seams such that the magnet wire is less likely to be exposed when the wrapped magnet wire is bent into a proper coil arrangement, the cost of multiple tape layers may be prohibitive for many applications.
Another known cigarette wrap type technique entails extruding onto the full circumference of a round conductor wire a heat activated adhesive. An electrically insulating wrap is then longitudinally wrapped around the entire adhesive coated wire to provide dielectric properties absent in the adhesive. A shortcoming of this technique is that because a complete wrapping of the wire is required to ensure that the wire, when eventually wound into a coil, is properly insulated, large amounts of expensive insulating tape are required.
U.S. Pat. No. 5,254,806 discloses a wire wrapping method that uses pressure sensitive adhesive on an insulating tape to wrap a magnet wire. The adhesive is covered by a release paper which is stripped from the tape immediately prior to application of the tape to the wire. A disadvantage of this method is the inconvenience of handling the release strip during wire formation. A further disadvantage is that the adhesive is flowed onto the tape and then covered with the release strip in a secondary process frequently performed by an outside supplier. The need for the precoated tape may be a source of delays and may further increase the cost of the manufacturing process.
In another method for insulating wire disclosed in U.S. Pat. No. 4,159,920, a binder coated tape is applied to a heated conductor wire. Besides requiring that heating units be provided to heat the wire before and possibly after tape application, this method further suffers from the need to use tape precoated with an adhesive, which as described above may increase the manufacturing costs of the wire.
In order to provide a magnet wire coil which utilizes less wrapped wire, one prior art design traversely winds a pair of magnet wires to form a layer. One wire of the pair is completely wrapped in insulation while the other wire is not provided with insulation. For this design, the turn to turn insulation is intended to be achieved by the insulation of the wrapped wire interposed between successive windings of the bare wire. This coil design may complicate manufacture as two wires must be handled during coil assembly.
Thus, it would be desireable to provide an insulated magnet wire which can be readily manufactured and which can serve to provide adequate insulation for a coil without wasting valuable raw materials.